In general, a hydraulic excavator as a representative example of a working machine is provided with an engine as a electric power source for traveling and working, wherein a hydraulic pump is driven by this engine. The hydraulic excavator operates hydraulic actuators such as a hydraulic motor and a hydraulic cylinder by pressurized oil supplied from the hydraulic pump to perform an excavating operation of earth and sand or the like.
On the other hand, there is known a hybrid-type working machine using both an engine and an electric motor together, as a working machine such as a hydraulic excavator. This hybrid-type working machine includes an engine, an electric motor that is driven by the engine to generate electric power or assists in a drive of the engine by supply of electric power thereto from an electricity storage device, the electricity storage device that stores electric power to be supplied to the electric motor, and an inverter for controlling an operation of the electric motor.
Here, the electricity storage device or the inverter mounted on the hybrid-type working machine is required to be used under an appropriate temperature condition. Therefore, the hybrid-type working machine is provided with a heat exchanger for cooling the engine, the hydraulic pump and the like, and besides, a cooling circuit including a radiator for cooling the electricity storage device and the inverter (Patent Document 1).